The State Department says targets could include tourist sites, restaurants, commercial centres and transportation.

The US has warned Americans on the possible threat of attacks in Europe this summer.

The State Department said on Tuesday targets could include tourist sites, restaurants, commercial centres and transportation.

"We are alerting US citizens to the risk of potential terrorist attacks throughout Europe, targeting major events, tourist sites, restaurants, commercial centers and transportation," it said in a travel alert expiring August 31.

"The large number of tourists visiting Europe in the summer months will present greater targets for terrorists planning attacks in public locations, especially at large events."